The invention is directed to a thyristor employing a semi-conductor element which includes a main thyristor and at least one auxiliary arrangement operative to amplify the control current, with the main thyristor having an emitter (termed the main emitter) and the auxiliary arrangement having also an emitter (termed the auxiliary emitter), a base being common to both the main thyristor and the auxiliary arrangement, with pn-junctions being disposed between the base and emitters, and provided with emitter electrodes disposed on the respective emitters and a control electrode which is connected to the base.
Thyristors of this type are known and, for example, have been described in German OS 1,639,019. The auxiliary arrangement has the function of amplifying the control currents supplied to the control electrode of the thyristor whereby it is possible to keep the control current low and at the same time apply a current to the main thyristor of sufficient magnitude for uniform ignition.
Experiments with infrared-photography have shown that in a thyristor of this type, the load current in the auxiliary arrangement initially flows in a linearly extended zone. However, in the course of a few tenths of a microsecond such linear zone is constricted to a substantially shorter, smaller area which must then temporarily conduct virtually the entire load current. In the case of a steep-gradient current rise of several 100 Al.mu.s, as controlled by the external circuitry, considerable heating takes place in such small area. In particular, at high repetition frequencies, destruction of the thyristor can result.